Lifespan of amps and preamps?

Never change a running system... But if you want to get an upgrade, you can also have a deeper look on used (but not vintage) equipment. Many of the high end gems I got over the last years were used but in great shape. Never forget that also loudspeakers are ageing over the years. And that technology advances. I recently tested the new Monitor Audio Gold 200 and it outclassed my Infinity Kappa 7 II Series 2 in almost every way you can imagine despite of being significantly smaller: more resolution, more precision and very clear bass. So visit some dealers with a good portfolio and proper showrooms, discover the 2020 high end world and enjoy!!!

Greetz, Hans

Addendum: I myself am into vintage for a long time. It can get you a lot of listening pleasure, but it can also cost you a lot of money for maintenance and repair. So think twice before buying "golden oldies"...

So long, Hans

I agree. It takes passion to own vintage gear. As an example, a newly produced Scott 340b volume pot off ebay costs around $100.

To OP, if you want to keep your system, you should at least do a recap, which I think is now a must. And as I said, some other maintenance like cleaning, checking values of resistors and other components. By experience though, electrolytics are almost always the critical issue. 

audite84 is right.  Electrolytic caps decay over time.  Even if the units are working, it's unlikely that they are at peak performance.  A 535 is a very good amp, and well worth having this done.  If you have been happy with the performance, it's probably more economical than buying new.  

Sounds like a nice sweet spot for you. I have an NAD 3155 (bought new in '85) which I still use as a pre. If it works for you, go with it unless it starts to sound bad, you can always renovate. That GFA 535 sounds like a keeper, too. I would say, you certainly got great value out of the system.